[OpenMP] Disable libomptarget profiling by default if built via the "runtimes" setup

[mstorsjo]

Summary

In the "runtimes" setup, the runtime (e.g. OpenMP) can be built for
a target entirely different from the current host build (where LLVM
and Clang are built). If profiling is enabled, libomptarget links
against LLVMSupport (which only has been built for the host).

Thus, don't enable profiling by default in this setup.

This should allow relanding D113253.

[protze.joachim]

openmp/CMakeLists.txt

60

For the cross-compilation use case, shouldn't the question be whether the code is compiled for Windows/MacOS rather than compiled on Windows/MacOS?

65

Can we check, whether the target architectures for llvm and runtimes are the same? In that case, building with profiling sounds reasonable.

[mstorsjo]

openmp/CMakeLists.txt

60

In CMake, APPLE and WIN32 refer to the target of the current cross compiler, not the host running the compilation. So you can do e.g. if (WIN32) # add windows specific sources endif() which is safe and correct wrt cross compilation.

65

I guess we could, but I'm afraid it could become brittle.

The situation that prompted this was that the main LLVM build and the runtimes disagreed on whether zlib was found, even if they were targeting the exact same OS/architecture. (They disagreed because the runtimes build used/will use a slightly different cmake setup, which makes the library probing behave slightly differently - also in general, runtime libraries shouldn't need to probe for external dependencies). As the main LLVM setup had zlib (which was a dependency of LLVMSupport) but the runtimes build didn't, libopenmptarget failed to include the LLVMSupport dependency.

I'm just suggesting changing the default here - one can still request it enabled (and see if it actually works or not).

The main issue as I see it, is that when building via the runtimes setup, using LLVMSupport from a runtime is a bit of a layering violation/inversion - the main overall direction is to untangle the runtimes from the main LLVM build as much as possible, that's at least what @phosek has suggested.

If building via LLVM_ENABLE_PROJECTS, so that the openmp runtime is built with the same compiler and configuration, alongside LLVM itself, using LLVMSupport is fine. Although, at least libcxx/libcxxabi/libunwind are entirely deprecating building them that way, but I'm not sure if openmp has quite as much desire to deprecate that setup.

[mstorsjo]

@phosek - What do you think about this? It’s needed to be able to reland D113253.

[mstorsjo]

In D114083#3162188, @mstorsjo wrote:

@phosek - What do you think about this? It’s needed to be able to reland D113253.

Ping @phosek - I'd like to get back on track for relanding D113253 one way or another...

[protze.joachim]

lgtm

[mstorsjo]

@AndreyChurbanov - is this ok with you?

[AndreyChurbanov]

In D114083#3176908, @mstorsjo wrote:

@AndreyChurbanov - is this ok with you?

Yes, I am OK with this.

openmp/CMakeLists.txt

set(OPENMP_TEST_FLAGS "" CACHE STRING
   "Extra compiler flags to send to the test compiler.")
 set(OPENMP_TEST_OPENMP_FLAGS ${OPENMP_TEST_COMPILER_OPENMP_FLAGS} CACHE STRING
   "OpenMP compiler flag to use for testing OpenMP runtime libraries.")
 
 set(ENABLE_LIBOMPTARGET ON)
 # Currently libomptarget cannot be compiled on Windows or MacOS X.
 # Since the device plugins are only supported on Linux anyway,
 # there is no point in trying to compile libomptarget on other OSes.
 if (APPLE OR WIN32 OR NOT OPENMP_HAVE_STD_CPP14_FLAG)
   set(ENABLE_LIBOMPTARGET OFF)
 endif()
 
+set(ENABLE_LIBOMPTARGET_PROFILING OFF)
+if (ENABLE_LIBOMPTARGET AND NOT LLVM_RUNTIMES_BUILD)
+  set(ENABLE_LIBOMPTARGET_PROFILING ON)
+endif()
+
 option(OPENMP_ENABLE_LIBOMPTARGET "Enable building libomptarget for offloading."
        ${ENABLE_LIBOMPTARGET})
 option(OPENMP_ENABLE_LIBOMPTARGET_PROFILING "Enable time profiling for libomptarget."
-       ${ENABLE_LIBOMPTARGET})
+       ${ENABLE_LIBOMPTARGET_PROFILING})
 option(OPENMP_ENABLE_LIBOMP_PROFILING "Enable time profiling for libomp." OFF)
 
 # Build host runtime library, after LIBOMPTARGET variables are set since they are needed
 # to enable time profiling support in the OpenMP runtime.
 add_subdirectory(runtime)
 
 # Build libompd.so
 add_subdirectory(libompd)